Transparent conductor-embedding high-sensitive germanium NIR photodetector

An extreme NIR photodetector was achieved by using transparent conducting layers on a small energy bandgap germanium (Ge) semiconductor. The optically-transparent and electrically-conducting layers (AZO and ITO) were sequentially coated on a Ge substrate by a large-scale available sputtering method, which spontaneously forms a Schottky junction (ITO/AZO/Ge) without an intentional doping process. The interface effect between AZO and Ge materials was studied at different depth levels from the surface using XPS and UPS measurements. The energy bands of AZO were bent at the interface to make a rectifying junction with Ge. The band bending at the interface is one of the important characteristics of Schottky photodiodes. The dark I-V profile of the AZO/Ge device showed rectification ratio of 22.36 along with the lowest reverse saturation current of 53.6  μA. The leakage current was perfectly suppressed due to the passivation effect of GeOx interfacial layer. At low frequency region, the Mott-Schottky analysis measured the barrier height as 0.38 eV which was correlated well with the value derived from the simulated band offset using SCAPS. This functionally designed transparent-embedding photodetector showed the excellent photoresponses for NIR regions. We propose a promising approach for highly-sensitive NIR photodetector by using a small energy bandgap Ge semiconductor with the transparent conducting layer.